Buck converters typically run in one of two modes: PFM (pulse frequency modulation) or PWM (pulse width modulation).
PFM mode is typically used for low load currents. In PFM mode the buck turns on a PMOS high-side switch when the output voltage falls below a reference voltage. The PMOS high-side switch is then turned off when the current in the coil reaches a threshold value (sleep current limit). A NMOS low-side switch is turned on when the PMOS high-side switch is turned off. The NMOS low-side switch is then turned off when the current in the coil is fully discharged. PFM mode is typically not used for large currents as the current limit is normally set low to maximize efficiency.
There is a general requirement for buck converters to have the output current measured—this measurement is often needed by the system in which the converter is integrated. Typically a buck only measures current only either on the PMOS or the NMOS. This means that the output current may only be measured either when the PMOS is on, or when the NMOS is on, and not all the time.
In prior art the output current of buck converters is measured in PWM mode only. A prior art sensing scheme is as follows: an LX voltage at a coil terminal is sensed during on-time of the PMOS high side-switch. The voltage is averaged over this time and, when the NMOS low-side switch is on, this average voltage is sampled instead of the LX voltage. Then the current is calculated from this sense voltage.
This current calculation is valid and no error is introduced in PWM mode. A long-term average of this current sense waveform correctly averages the output current of the buck.
The scheme discussed above does NOT apply during discontinuous PFM mode. Here the current waveform takes the form of a series of triangular pulses, with large periods of no-current between them.
If the average is taken during the PMOS on time, and is it then assumed whenever the PMOS is off, the average current will be much too high, as during a large period the coil current is actually zero.
Other schemes can be created where the current is sampled during the PMOS on-time, zero current is sampled during the time when neither pass-device is on, and then the average is used during the NMOS on-time. These schemes either add an error during the NMOS on-time, or are complex to implement.
In general, measuring the output current in DCM PFM is not practical with existing solutions.
It is a challenge to designers of DC-to-DC converters to find a method to measure the output current of a buck converter during PFM mode.